The PiPET (Projection Imaging/Positron Emission Tomography) rodent imaging system completed last year continued to be upgraded as experience with this system accumulated. An improved user interface was created that allows uncomplicated initiation and processing of PiPET imaging studies conducted in both the projection and rotational tomography modes. Further modifications are now under development that should improve the performance of this prototype substantially: completely new, fast HV dividers; preampli-fiers; signal divider chains for the R3941 PSPMT; new two-layer, fast DOI crystal arrays; appropriate shielding to reduce out-of-field activity effects; etc. Despite the simplicity and low cost of this system, it continues to find numerous applications in studies of mice and rats. These studies include measurements of hypoxia in human tumor xenografts, changes in organ function due to genetic manipulation, and the consequences of attempts to reverse these changes with in vivo gene therapy. Investigations continued to identify optimal subsystem designs for a stationary PET ring scanner that has been in the planning stage for the past several years. Work performed during this period demonstrated that multiple position-sensitive photomultiplier tubes could be combined through common resistive signal divider chains without sacrificing image quality, thus reducing the number of signal lines and the cost by a large factor. Several high-speed data acquisition boards were also evaluated for potential inclusion in this system. One of these boards appears to meet virtually all system requirements including very high data rates (> 500,000 events/sec), multiple data channels, and low cost. Exhaustive studies were also undertaken to evaluate a variety of optical glues for fabricating the DOI crystal arrays in this system. Finally, in collaboration with Thomas Jefferson National Laboratory, high-speed preamplifiers, new HV dividers, and resistive signal dividers were devised for the R5900 PSPMTs for use in this new high- performance system soon to be fabricated. - Small Animal PET, Small Animal SPECT, Small Animal Radionuclide Imaging